A footing's size is fixed by one simple rule: the pressure it puts on the soil must not go over the soil's safe bearing capacity. Let's use that rule directly instead of jumping straight to a formula.
The column brings down a load of $225\ \text{kN}$. The soil under the footing can only carry $100\ \text{kN}$ on every square metre of contact area. So the footing needs at least enough base area that spreading $225\ \text{kN}$ over it works out to no more than $100\ \text{kN/m}^2$.
Required area:
\[ A = \frac{225}{100} = 2.25\ \text{m}^2 \]Since the footing is square, both sides are equal; call the side $L$. Then $L \times L = 2.25$, so:
\[ L = \sqrt{2.25} = 1.5\ \text{m} \]Check: a $1.5\ \text{m} \times 1.5\ \text{m}$ footing has an area of $1.5 \times 1.5 = 2.25\ \text{m}^2$, and the pressure it would feel is $225 / 2.25 = 100\ \text{kN/m}^2$, exactly equal to the soil's safe limit, not more. So this side length is the minimum size that keeps the footing safe.
So the minimum side length of the square footing is 1.5 m.